This invention relates to radio frequency systems and more particularly to a radio frequency antenna input and filter assembly for a tuner wherein the chassis is coupled to an alternating current source for supplying operating power to the receiver without the benefit of a transformer (hot chassis). The assembly provides isolation sufficient to guarantee safety from electrical shock for an operator of the receiver contacting metallic terminals and/or adjustment knobs on the receiver and also reduces the amount of undesirable signal coupled to the tuner from external electromagnetic radiation.
One of the terminals which are typically accessible to the operator of the receiver is the antenna terminals. Various antenna cable and cable coupling devices are used to transmit desirable signals from an antenna to the tuner. Among these types is a shielded coaxial cable having a characteristic impedance of 75 ohms. The 75 ohm coaxial cable has two concentric conductors (i.e., an inner conductor and an outer or shield conductor). When the 75 ohm coaxial cable is employed, the outer conductor of the coaxial cable is connected to the hot chassis of the receiver to provide an effective shielding from external undesirable electromagnetic fields and a relatively high impedance as compared to the source impedance of the alternating current source to prevent electrical shock to a person contacting the cable or antenna terminals.
Prior art apparatus of a coaxial cable coupled to the antenna terminals of a hot chassis television receiver wherein feedthru capacitors are used, are U.S. Pat. No. 3,913,038 of Carter et al. and U.S. Pat. No. 4,267,529 of Brunet al. The Carter et al. arrangement utilizes such a feed-thru capacitor with a shielded coaxial cable having a braided outer conductor, and ferrite beads circumposed about the braid for producing an input filter in combination with capristor devices. An antenna isolation assembly must isolate the "hot-chassis" from the antenna connector, and provide rejection of co-channel interference by eliminating as much as possible, any ingress to the tuner of signal, of extraneous signals. However, this arrangement requires a costly high voltage rated feed-thru capacitor arrangement to shunt unwanted signals from the coaxial antenna input terminal to ground, and presents production difficulties, because when the cable length is small, there is not sufficient friction between the braid and the center insulation to provide a stable mechanical arrangement. This is particularly awkward since the braid, with a ferrite bead mounted thereon, can slide with respect to the center insulation during assembly. This lack of mechanical stability increases the costs and difficulties of production. Accordingly, it is desirable to provide a shield for an input cable which can accept a ferrite bead while maintaining a stable mechanical arrangement.